Dehydration

Dehydration during exercise is a compound event. A major issue can be thought of volume depletion. This means that the serum (part of blood that isn’t cells) volume is slightly reduced in most people simply because just walking around we can get away with that. Another part of this is that during regular activity, the blood flow to muscles is relatively small. However, when you exercise, blood flow increases dramatically to the muscles (read pumped up). In cardiovascular physiology, the major mode of thinking is to address the issue of “where is the blood?” And this question is usually applied from the point of view of the heart. In this case, volume flow to muscles takes away from the rest of the body. The body compensates by reducing flow to the organs least necessary during exercise (e.g. the stomach). This is normally enough to compensate for exercise but we go hard. Now, blood flow through the heart is modified but ultimately depends on the return of blood from the body to supply what is needed for the next beat. Now we add the final part of the problem – sweat. This reduces further blood volume and this time it specifically lowers the serum volume resulting in more viscous blood. The increased viscosity of the blood reduces the efficiency of the heart. So now we have “stolen” the blood from every source and continue to make the heart work harder. Then Sensei asks for more and your mind can ignore 5% more pain. Blood flow everywhere begins to be reduced ever so slightly but at nearly the instant that you make another strong exertion, you increase your abdominal pressure, let out a good kiai, focus and then relax. Result? You change the volume of blood returning to the heart and the ensuing beat(s) contains a relatively small amount of blood. You don’t give the brain as much blood as normal and the next thing you know, you are looking at the ceiling wondering if those people in white are angels and trying to figure out why clouds are so hard.

How do you know when you are dehydrated? Unfortunately, thirst itself may not be enough. When you sweat a lot you always want to make sure you drink a lot, even if you are not thirsty. Also, strong color and smell of urine is a good indication of dehydration (the kidneys retain more water and urine is more concentrated). Stronger symptoms are headache, mind fogginess and weakness. Eventually, you [can] lose consciousness. I would assume that less oxygen reaches the brain (and other body parts) at that stage due to blood thickening, but I won’t sign my name to it…

Avoiding dehydration is simple enough: drink frequently. Large amount of water at one time isn’t good enough: most of the water will “flow through” without being absorbed. Drink much after a workout, but keep drinking every hour or so for a while, until you’ve restored all your liquid (urine is clear).

What I would like to add concerns the effects on the kidney. Indeed, as I recall (and I could have the state wrong) it was either the Massachusetts or Pennsylvania state troopers that were involved, and the reason they changed their policy of exercising people until they dropped is because someone finally died from renal failure.

When you exercise vigorously, it is normal to have “some” muscle destruction taking place. The muscle cells are lysed (broken up) and their contents (electrolytes, myoglobin, enzymes) circulate in the bloodstream. The more dehydrated you are, the higher in concentration these entities become. It is “normal” if you examine someone who’s been exercising urine under a microscope to see a few red cells (blood). It is decidedly NOT normal to see either blood macroscopically or “smoke” colored urine. Smoke-colored urine results from either the breakdown of blood (hemoglobin) or muscle (myoglobin). The enzymes which are released with the breakdown of the muscle cells is the most destructive part, and can cause damage to the kidney, even to the point of causing kidney failure. This is more likely to occur when the person is dehydrated. The kidney failure is usually temporary (1-7 days), but it is a serious process, and as mentioned above, some people die from this.

The “take home message” is just to use some common sense, and practice what all the sports magazines tell runners, bikers, and other athletes to do. Drink plenty of water before you exercise, and continue to drink during exercise (3-5 cups of water/hour for an average sized person). Electrolyte mixes (gatorade, powerade, and the like) have only a marginal advantage over the best drink on the planet: water.

This works great if you have access to a scale. (Sensei, may I be excused to go weigh myself?)

Dehydration is not shock. Shock is the global failure of cellular oxidative metabolism. One form of shock is hypovolemic shock in which the circulating blood volume is low. Dehydration may cause shock but is hardly synonymous with shock. Other forms of shock may have normal blood volume but the volume is maldistributed, e.g. anaphylactic shock. In some cases of shock the blood volume may in fact be increased but the heart may be unable to circulate the blood and oxygen to the tissues, e.g. cardiogenic shock. They just aren’t the same thing at all.

Dehydration is the loss of total body water. Hypovolemia is a low circulating blood volume. Total body water amounts to about 0.6 liters per Kg body weight. For the prototypical 70kg person that means a total body water of 42 l. About 2/3 of this is in the cells (intracellular water) or about 28 l. About 1/3 is outside of the cells (extracellular water – 14l). About 2/3 of the extracellular water (9-1/3 l.) is in the space between the cells (interstitial fluid). About 1/3 (4-2/3 l.) is in the vascular space (intravascular water). Most of the time when we lose fluid we lose fluid from the intravascular space. Sweat glands for example take fluid from the blood and put it on the skin. The kidneys take fluid from the blood and produce urine. If we vomit or have diarrhea we lose fluid that the stomach, pancreas, small bowel etc. has put into the gut lumen from the blood stream. Thus, most of our fluid losses are from the intravascular space.

If our fluid loss is slow then our body can move fluid, first from the interstitial space and then more slowly from the intercellular space, into the intravascular space to maintain the intravascular volume. If we lose the fluid suddenly then there is no time to accomplish this. A good rule of thumb is that we can mobilize about 1-2 l. per day maximum from the interstitial and intercellular spaces into the intravascular space. If I cause you to get dehydrated slowly (i.e., I put you in a warm environment and don’t give you free access to water but yet you have some water) You will not have symptoms till you have lost about 5-10% of your total body water (2.1 l. – 4.2 l.). At that time if you stand up suddenly your hart rate will go up but your blood pressure will remain normal. If you lose 10%-15% (4.2 l. – 6.3 l.) then when you stand up your blood pressure will go down and you may become lightheaded. If you lose 15-20% (6.3 l. – 8.4 l.) then your blood pressure will be down even lying down. At that time you would probably meet diagnostic criteria for hypovolemic shock.

Of course, if I suddenly take fluid out of your intravascular space and give you no time to mobilize fluid from the other compartments into the intravascular space you will have symptoms at lower levels of fluid loss. That is why some people get lightheaded when they stand up after donating 0.5 l of blood. All that fluid (about 0.25 L. fluid and 0.25 l. of blood cells) came straight out of the vascular space in a period of a few minutes.

Most of the fluid we lose each day is in the form of urine. Healthy people with normal hearts, livers and kidneys make at least 0.5 ml of urine per Kg body weight per hour (0.012 L./Kg/day). We also lose some fluid each day in stool, in sweat and in our breath. This amounts to about 0.5 l per day. In hot conditions, when we have a fever, or when we exercise, this goes up. This is the principle way we become dehydrated in the dojo. We need to replace this fluid loss. We also need to replace the sodium and potassium and other minerals which we lose in sweat (we lose little in the way of minerals in our breath). When we become hypovolemic several things happen. First our heart speeds up. It pumps less volume per beat (stroke volume) but beats more often to compensate for this and maintain cardiac output (the number of liters of blood pumped per minute). Second the blood levels of catecholamines (epinephrine, norepinephrine and dopamine) go up. This causes blood vessels in the critical organs such as heart, brain, liver and kidneys to dilate and the vessels in the less critical areas (gut, muscles) to constrict. Changes in the output from our sympathetic nervous system reinforce this. The net effect is reduced blood flow to the muscles and gut and more to the brain etc. Thirdly, receptors that measure our blood pressure, our sodium level, etc. tell the brain to increase our volume, this kicks in our thirst mechanism. Fourthly, the atria of our heart, since they are smaller in diameter reduce their production of ANP, a peptide hormone that stimulates the kidneys to make urine. Thus the kidneys reduce their secretion of sodium and urine.

Training without replacing our fluid and electrolyte losses doesn’t condition us. It doesn’t make us tougher. It doesn’t improve our endurance. When we get tachycardic and thirsty our body is telling us something. We need to listen to it.